ELECTRON CORRELATIONS IN DISORDERED ALLOYS AND AT METALLIC SURFACES

摘要

The density functional theory (DFT) supplemented by the local density approximation (LDA) or generalized gradient approximation (GGA) in which the electron-correlation part is treated on the basis of the electron gas model is a highly reliable method for evaluation of the ground state properties of molecules and solids. However, the DFT fails in some cases, e.g., for excitation spectra of solids or in the evaluation of the gap in insulators and semiconductors and also for solids (such as lanthanides and actinides or transition-metal oxides) whose electronic structure is better described in terms of atomic-like electronic states rather than in terms of the electron gas model on which the LDA and GGA are based. The electronic properties of strongly correlated materials which cannot be adequately described within the DFT are usually studied in the framework of simplified models like, e.g., the single-band Hubbard model. The strength of correlation effects is usually classified in terms of the ratio of the on-site Coulomb energy U to the bandwidth w. One can distinguish three regimes, namely, (ⅰ) the weak interaction case (U/w < 1, transition metals), (ⅱ) the intermediate interaction case (U/w ≈ 1, metal-insulator transition regime and Kondo systems), and (ⅲ) the strong interaction case (U/w > 1, rare-earth systems and wide gap solids). It should be mentioned that a general approach bridging all the above cases is still missing even at the model level. Recently some progress has been made in extending the validity of the DFT, for example the time-dependent LDA can yield the excitation energies, and proper account of the self-interaction corrections (SIC) lead to a remarkable improvement for rare-earths. The so-called GW approximation (GWA) was successfully applied to the gap problem in a number of semiconductors and insulators as well as to studies of photoemission spectra of metallic systems. An alternative approach to the gap problem which is based on a straightforward generalization of the LDA is the LDA+U method which also yielded band narrowing in the photoemission spectra of nickel, but it failed to produce the well-known satellite structure below the main peak. The presence of the satellite in Ni was successfully explained in the framework of the T-matrix formalism developed on ab initio level (cf. Ref. 10). Another approach to the satellite and the band narrowing in nickel is based on a three-body scattering approximation that employs the Faddeev equations. The dynamical mean-field theory (DMFT) aims at accurate solution of the correlation problem on a single site which is coupled to the rest of the lattice in a selfconsistent way. The DMFT, besides many studies on the model level, was also applied to realistic calculations of correlated systems, e.g., δ-Pu.